From the OpenGL ES 3.1 spec:

11.1.3.11 Validation
It is not always possible to determine at link time if a program object
can execute successfully, given that LinkProgram can not know the state
of the remainder of the pipeline. Therefore validation is done when the
first rendering command which triggers shader invocations is issued, to
determine if the set of active program objects can be executed.

For draws, this CL moves the PPO link operation to ValidateDrawStates()
to generate PPO link failures within ANGLE's validation layer, so we
fail any rendering commands during command validation.

For dispatch, PPOs are linked during Context::prepareForDispatch(),
where the PPO is converted from draw to compute, since that conversion
requires a re-link. This re-link shouldn't fail due to errors that would
have been caught during validation, since the compute shader must have
successfully linked before it can be included in the PPO in the first
place. We don't re-link when converting back to draw, since it's
possible there are validation errors (which we want to catch during
validation of the next rendering command).

Bug: angleproject:5064
Test: dEQP.GLES31/functional_separate_shader_validation_es31_*
Test: ContextNoErrorTest31.DrawWithPPO
Test: ProgramPipelineTest31.VerifyPpoLinkErrorSignalledCorrectly
Change-Id: Ibb249e893c007a83cc6b813f848a660bfa34ecb0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2422375
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Ian Elliott <ianelliott@google.com>